Electrochemical PEM cells, and particularly, PEM fuel cells are well known. PEM fuel cells convert chemical energy to electrical power with virtually no environmental emissions and differ from a battery in that energy is not stored, but derived from supplied fuel. Therefore, a fuel cell is not tied to a charge/discharge cycle and can maintain a specific power output as long as fuel is continuously supplied. The large investments into fuel cell research and commercialization indicate the technology has considerable potential in the marketplace. However, the high cost of fuel cells when compared to conventional power generation technology has deterred their potentially widespread use. Costs of fabricating and assembling fuel cells can be significant, due to the materials and labor involved, and as much as 85% of a fuel cell's price can be attributed to manufacturing costs.
A single cell PEM fuel cell consists of an anode and a cathode compartment separated by a thin, ionically conducting membrane. This catalyzed membrane, with or without gas diffusion layers, is often referred to as a membrane electrode assembly (“MEA”). Energy conversion begins when the reactants, reductants and oxidants, are supplied to the anode and cathode compartments, respectively, of the PEM fuel cell. Oxidants include pure oxygen, oxygen containing gases, such as air, and halogens, such as chlorine. Reductants, also referred to herein as fuel, include hydrogen, natural gas, methane, ethane, propane, butane, formaldehyde, methanol, ethanol, alcohol blends and other hydrogen rich organics. At the anode, the reductant is oxidized to produce protons, which migrate across the membrane to the cathode. At the cathode, the protons react with the oxidant. The overall electrochemical redox (reduction/oxidation) reaction is spontaneous, and energy is released. Throughout this reaction, the PEM serves to prevent the reductant and oxidant from mixing and to allow ionic transport to occur.
Current state of the art fuel cell designs comprise more than a single cell, and in fact, generally combine several MEAs, flow fields and separator plates in a series to form a fuel cell “stack”; thereby providing higher voltages and the significant power outputs needed for most commercial applications. Depending on stack configuration, one or more separator plates may be utilized (referred to as a “bipolar stack”) as part of the stack design. Their basic design function is to prevent mixing of the fuel, oxidant and cooling input streams within the fuel cell stack, while also providing stack structural support. These separator plates serve as current collectors for the electrodes and may also contain an array of lands and grooves formed in the surface of the plate contacting the MEA, in which case the separator plates are often referred to only as “bipolar plates” and the array of lands and grooves as “flow fields”. Alternatively, the flow field may be a separate porous electrode layer. Ideal separator plates for use in fuel cell stacks are thin, lightweight, durable, highly conductive, corrosion resistant structures that can also, if desired, provide effective flow fields and thereby become bipolar plates.
In the flow fields, the lands conduct current from the electrodes, while the grooves between the lands serve to distribute the gaseous reactants utilized by a fuel cell, such as hydrogen, oxygen or air, evenly over the faces of the electrodes. The channels formed by the lands and grooves also facilitate removal of liquid reaction byproducts, such as water. A thin sheet of porous paper, cloth or felt, usually made from graphite or carbon, may be positioned between each of the flow fields and the catalyzed faces of the MEA to support the MEA where it confronts grooves in the flow field to conduct current to the adjacent lands, and to aid in distributing reactants to the MEA. This thin sheet is normally termed a gas diffusion layer (“GDL”), and is incorporated as part of the MEA.
Fuel cell stacks may also contain humidification channels within one or more of the coolant flow fields. These humidification channels provide a mechanism to humidify fuel and oxidants at a temperature as close as possible to the operating temperature of the fuel cell. This helps to prevent dehydration of the PEM as a high temperature differential between the gases entering the fuel cell and the temperature of the PEM causes water vapor to be transferred from the PEM to the fuel and oxidant streams. The location of the humidification channels can either be upstream from the MEA, such as in the fuel cell stacks described in U.S. Pat. No. 5,382,478 to Chow et al., and U.S. Pat. No. 6,066,408 to Vitale et al., or downstream from the MEA, such as those described in U.S. Pat. No. 5,176,966 to Epp et al.
Of necessity, certain stack components, such as the GDL portion of the MEA, are porous in order to provide for the distribution of reactants and byproducts into, out of, and within the fuel cell stack. Due to the porosity of elements within the stack, a means to prevent leakage of any liquid or gases between stack components (or outside of the stack) as well as to prevent drying out of these porous elements due to exposure to the environment is also needed. To this end, gaskets or other seals are usually provided between the surfaces of the MEA and other stack components, such as flow fields, and on portions of the stack periphery. These sealing means, whether elastomeric or adhesive materials, are generally placed upon, fitted, formed or directly applied to the particular surfaces being sealed. These processes are labor intensive and not conducive to high volume manufacturing and add to the high cost of fuel cells. The variability of these processes also results in poor manufacturing yield and device reliability.
Fuel cell stacks range in design depending upon power output, cooling, and other technical requirements, but may utilize a multitude of MEAs, seals, flow fields, and separator plates, in intricate assemblies that result in manufacturing difficulties and further increase fuel cell costs. For example, one fuel cell stack, described in U.S. Pat. No. 5,683,828, to Spear et al., employs bipolar plates containing up to ten separate layers adhesively bonded together, each layer having distinct channels that are dedicated to passing cooling water through the fuel cell stack for thermal management.
These multitudes of individual components are typically assembled into one sole complex unit to form the fuel cell stack. The stack is then compressed, generally through the use of end plates and bolts although banding or other methods may be used, such that the stack components are held tightly together to maintain electrical contact there between. These current means of applying compression add even more components and complexity to the stack and pose additional sealing requirements. Various attempts have been made in the fuel cell art to cure these deficiencies in fuel cell stack assembly design and thereby lower manufacturing costs.
U.S. Pat. No. 6,080,503, to Schmid et al., describes the replacement of gasket based seals within certain portions of the stack with an adhesive based material in the form of tapes, caulks or layers. However, assembly of this stack still requires manual alignment of the components during the adhesion process, in a manner not unlike caulking a seal, and sealing only occurs at those interfaces where adhesive has been applied through active placement.
U.S. Pat. No. 4,397,917, to Chi et al., describes the fabrication of subunits within a fuel cell stack for ease in handling and testing. However, this design relies on conventional sealing among the components and between subunits. In addition no manifolds internally penetrate the subunit.
U.S. Pat. No. 5,176,966, to Epp et al., describes a method of forming at least some of the required gaskets directly into the fuel cell stack assembly. Specifically, the MEA is made with corresponding carbon paper and then an extrudable sealant is applied into grooves cut within the carbon paper.
U.S. Pat. No. 5,264,299, to Krasij et al., describes a fuel cell module having a PEM interposed between the two porous support layers which distribute reactant to the catalyst layers in which the peripheral portion of the support layers are sealed with an elastomeric material such that the PEM is joined with the support layers and the open pores of the support layers are filled with the elastomeric material making it fluid impermeable. The elastomeric material solidifies to form a fluid impermeable frame for the PEM and support layer assembly.
U.S. Pat. No. 5,523,175, to Beal et al., describes an improvement of U.S. Pat. No. 5,264,299 which comprises a plurality of gas distribution channels on the support layers and utilizes a hydrophilic material for sealing of the open pores. However, this improvement does not address the issue of gaps between the MEA and the support plates.
U.S. Pat. No. 6,165,634, to Krasij et al., describes the use of a flouroelastomer sealant in bonding individual stack components and the edges of several cells within a stack. However, this improvement requires piece-meal application to the components and, as such, does little to improve the labor required to assemble the stack.
U.S. Pat. No. 6,159,628, to Grasso et al., describes the use of thermoplastic tape as a replacement for traditional elastomeric gasket based seals thereby eliminating the waste associated with cutting gaskets from large sheets of elastomer. Unfortunately, similar to conventional sealing mechanisms, this method also requires manual placement of the tape pieces.
As can be seen from the above discussion, none of these designs adequately compensate for the current design deficiencies that result in the high manufacturing costs of fuel cell stacks. An improved style of fuel cell stack that is less complex, more reliable, and less costly to remove, replace and manufacture would be a significant addition to the field.
Accordingly, it is an object of the present invention to provide an improved fuel cell stack design which would assemble together individual modules to form a fuel cell stack of requisite power output, and would allow for disposal and replacement of an individual module in the event of a failure within one such module.
Another object of the present invention provides a fuel cell stack comprised of pre-fabricated individual modules that are standardized to specific power outputs or other technical specifications thereby allowing for the quick and efficient assembly of a complete fuel cell stack with minimal manufacturing processes being employed, by combining such standardized modules to meet the required specifications of the completed fuel cell stack.
Yet another object of the present invention is to provide for a reduction in the complexity of a fuel cell stack by reducing the number of components and seals required for stack construction, while maintaining the required power output for the stack, thereby increasing the reliability of the fuel cell stack.
Still another object of the present invention is to provide for an improved method of sealing porous components within the stack or a module thereof, as well as a method of sealing the stack or module periphery that is less labor intensive and more suitable to high volume manufacturing processes.
Still another object of the present invention is to provide a simplified compression means for the fuel cell stack assembly wherein the components of the fuel cell stack assembly would remain in close contact with a minimum of additional elements being added to the assembled stack.
Additional objects, advantages and novel features of the invention will be shown in the accompanying drawings and description.